In applying plastic insulating material to a wire to be insulated, the plastic material is delivered to a crosshead device which applies the insulating material to the wire by means of an extruder. The extruder includes a screw which forces the plastic material against a breaker plate having a plurality of holes therein. The plastic material is extruded through the holes in the breaker plate into a conduit or adapter through which the plastic flows into the crosshead device. The breaker plate functions to create a back pressure in the extruder and to shear the plastic material as it passes therethrough.
An extruder having a prior art breaker plate is disclosed in U.S. Pat. No. 3,121,255. The prior art breaker plates typically comprised a flat disc having a number of holes of equal diameter, with each hole having a constant diameter along its length. The consequence of this configuration is that there are a number of flat surfaces between the holes on the side of the breaker plate facing the screw. These flat surfaces provide "dead spots" on which the plastic insulating material tends to accumulate, decompose with time and temperature, and eventually smolder or burn. This degraded material is ultimately passed through the breaker plate with the plastic flow and contaminates the insulating layer applied by the crosshead. In addition, these prior art breaker plates impart a relatively high shear force to the plastic material which is detrimental to certain shear sensitive plastics, such as polyvinylchloride (PVC), polybutylterephthalate (PBT), and vulcanized insulated polypropylene (VIP). Because the holes are of uniform diameter and profile, there is a tendency for a greater volume of the plastic material to flow through the radially outermost portions of the breaker plate than the radially inner portions because the aggregate hole surface area is greater in the outermost portions than in the inner portions. This results in an uneven flow distribution into the crosshead which can affect the uniformity of the insulation layer applied to the wire.
Moreover, the uneven flow distribution through the breaker plate adversely affects the efficiency of the system since the buildup of degraded material requires more frequent cleaning of the breaker plate, thereby limiting the effective production cycle between cleanings. In addition, as mentioned above, the relatively high shear forces presented by the dead spaces on the breaker plate and the constant diameter holes pose a potential detrimental effect on certain shear-sensitive plastics.